1. Field of the Invention
The present invention relates to an image forming apparatus having an image writing device for writing a latent image on a surface of a photoconductor by deflecting a light beam by way of an optical defector and exposing the surface of the photoconductor with the deflected light beam, and more particularly to an image forming apparatus in which adjustment of a scanning direction of the light beam so as to be at a right angle relative to a direction in which a surface of the photoconductor moves such that a rectangular latent image is easily formed.
2. Discussion of the Background
It is well known that digital copying machines using electrophotography include an image writing device for writing a latent image on a surface of a photoconductor by deflecting a light beam by way of an optical defector and exposing the surface of the photoconductor with the deflected light beam.
FIG. 1 illustrates an example of such a digital copying machine. In FIG. 1, digital copying machine 100 includes an image reading device 111, a printing device 112 and an automatic document feeding device 113. The automatic document feeding device 113 separates each of the original document sheets set in the automatic document feeding device 113 and feeds the separated sheets to a contact glass 114 into a reading position. After an image of the original document is read, the automatic document feeding device 113 discharges the sheet from the reading position on contact glass 114.
FIG. 2 is a sectional drawing illustrating image reading device 111. Image reading device 111 includes a first carriage A having a light source including an illuminating lamp 115, a reflecting mirror 116, and a first mirror 117 (shown in FIG. 1). A second carriage B of image reading device 111 includes a second mirror 118 and a third mirror 119. When reading an original document placed on contact glass 114, first carriage A travels at a constant speed while second carriage B travels at a speed half that of first carriage A, thereby optically scanning the original document on the contact glass 114. The original document on contact glass 114 is illuminated by illuminating lamp 115 and reflecting mirror 116. As shown in FIG. 1, the light reflected by the original document is directed to a charge-coupled device (CCD) 122 by a lens 121 via first mirror 117, second mirror 118, third mirror 119, and a color filter 120. CCD 122 converts the received light image to electrical signals and outputs analog image signals representing the image of the original document. Referring again to FIG. 2, after the image of the original document is read, first carriage A and second carriage B return to respective home positions (represented by the solid line depictions) from the positions where image reading ends (represented by the line-and-dot line depictions). CCD 122 may be configured such that three arrays of CCD elements are arranged for R (red), G (green) and B (blue), respectively, for reading a original color document.
Referring now to FIG. 1, the analog image signals output from CCD 122 are converted into digital image signals by an analog-to-digital converter (not shown) and various kinds of image processing, such as converting multi-value data into binary data and vice versa, gradation level conversion, magnification ratio change, image editing and so on, are applied to the digital image signals by an image processing circuit included on image processing board 123.
In order to prepare photoconductor drum 125 to receive a latent image, photoconductor drum 125 is driven by a drive unit (not shown) and the surface of the photoconductor drum 125 is uniformly charged by a charging device 126. After the digital image signals have been processed with image processing board 123, they are sent to a semiconductor circuit board (not shown), and a latent image is formed on the surface of photoconductor drum 125 according to the digital image signals with an image exposure operation performed by a laser beam scanning device 127. The latent image on the photoconductor drum 125 is then developed with toner to form a visible toner image by developing device 128.
A transfer sheet is fed to a registration roller 136 from a selected one of sheet cassettes 133, 134 and 135, toward the photoconductor drum 125 at a timing to register the leading edge of the transfer sheet with the leading edge of a toner image formed on the surface of the photoconductor drum 125. The toner image on photoconductor drum 125 is transferred onto the transfer sheet with transfer device 130. The transfer sheet carrying the toner image is separated from the photoconductor drum 125 with separating device 131 and is conveyed by conveying device 137 to fixing device 138, where the toner image is fixed onto the transfer sheet. The transfer sheet carrying the fixed toner image is then discharged onto an exit tray 139. The surface of the photoconductor drum 125 is cleaned with cleaning device 132 after the transfer paper is separated such that residual toner is removed from the surface of photoconductor drum 125.
FIG. 3 is a schematic drawing illustrating the construction of the laser beam scanning device 127 and the relative positions of scanning device 127 and photoconductor drum 125. Laser beam scanning device 127 includes a semiconductor laser unit 140 having a semiconductor laser. A laser beam light emitted from the semiconductor laser is converted to a parallel light flux with a collimate lens (not shown) in the semiconductor laser unit 140. The parallel light flux is then reformed to a predetermined shape by passing through an aperture (not shown) in the semiconductor laser unit 140. The reformed light flux is converged in the sub scanning direction with cylindrical lens 141 and is thereby directed onto a surface of a polygonal mirror 142. Polygonal mirror 142 is formed in a polygonal shape and is rotated at a constant speed in a fixed direction with motor 143 (illustrated in FIG. 1). The rotation speed of the polygonal mirror 142 is determined according to the rotation speed of the photoconductor drum 125, the writing resolution of the laser beam scanning device 127, and the number of surfaces of the polygonal mirror 142.
The laser beam directed onto the polygonal mirror 142 is deflected by a reflecting surface of the polygonal mirror 142 at an equiangular velocity and is thereby directed into an f.theta. lens 144. The f.theta. lens 144 is configured to convert the laser beam deflected by the polygonal mirror 142 at equiangular velocity so as to scan the surface of the photoconductor drum 125 at a constant linear velocity. The laser beam is directed to the surface of the photoconductor drum 125 via reflecting mirror 145 and dust-proof glass 146. The f.theta. lens 144 also has a surface tilt correcting function, which causes a portion of the laser beam passing through f.theta. lens 144 at positions out of the image forming area, to be reflected by a synchronization detection mirror 147 so as to be detected by a synchronization detection sensor 148. The synchronization detection sensor 148 outputs a detect signal in accordance with the collision of the laser beam thereupon such that a synchronization signal for aligning a writing start position for each scanning in the main scanning direction (indicated by an arrow in FIG. 3) is obtained.
Laser beam scanning device 127, as described above, is attached to a housing of a main body of the digital copying machine 100 such that a latent image is formed on the surface of the photoconductor drum 125 with the laser beam in a rectangular form having a right angle at each comer, i.e., when a rectangular latent image formed on the surface of the photoconductor drum 125 with the laser beam and the latent image is developed with toner, one side of the rectangular toner image is perpendicular to the direction in which the photoconductor drum 125 rotates (parallel to the axis of the photoconductor drum 125) and the other side of the rectangular toner image is parallel to the direction in which the photoconductor drum 125 rotates (perpendicular to the axis of the photoconductor drum 125), or when the rectangular toner image is transferred onto a transfer sheet, sides of the rectangular toner image are parallel to edges of the transfer sheet respectively or each line extending from an end of a side of the rectangular image is perpendicular to an edge of the transfer sheet. The above feature that an image is formed in a rectangular form is realized if the laser beam of laser beam scanning device 127 scans the surface of photoconductor drum 125 in a direction substantially perpendicular to the direction in which photoconductor drum 125 rotates. Laser beam scanning device 127 is therefore attached to the housing of the main body of the digital copying machine such that the positional relationship between the laser beam scanning device 127 and the photoconductor drum 125 is set and fixed such that the scanning direction of the laser beam is substantially perpendicular to the direction in which the photoconductor drum 125 rotates so that a latent image is formed in a rectangular form. In some image forming apparatuses, relevant parts, such as for example a mirror, are made adjustable, such that, when the apparatus is shipped from the assembling factory, the scanning direction of the laser beam can made perpendicular to the direction in which the photoconductor drum rotates so as to form an image in a rectangular form by way of adjusting the relevant parts.
However, even when a laser scanning device is attached to a housing of a main body of an image forming apparatus such that a desired positional relationship between the scanning device and a photoconductor drum for making the scanning direction of the laser beam perpendicular to the rotation direction of the photoconductor (so as to forming an image in a rectangular form) is accomplished as described above, such a positional relationship may be lost during transportation of the apparatus to a user's cite or due to the condition of the place where the apparatus is placed at the user' site. In particular, when the apparatus is placed on an uneven or inclined plane, thereby distorting the apparatus, the desired positional relationship between the laser beam scanning device and the photoconductor drum may be lost. For example, the surface of the photoconductor drum may be deviated from the position where an image is formed in a rectangular shape with a laser beam of the laser beam scanning device, and thereby, the image is formed, for example, in a parallelogram.
Further, because the laser beam scanning device is generally located at the rear side of the apparatus and is packaged in a tightly sealed construction for preventing optical elements which are included in the scanning device being stained with dust, even if optical elements of the scanning device, such as mirrors, are made to be adjustable as above, proper adjustment of such optical elements is not easily accomplished at a user's site. Furthermore, addition of such adjusting feature increases the cost of the apparatus.